Time Measurement Units are used to measure the time between two waveform signals. The time between two waveform signals are useful in that this information can be used in various ways. In U.S. Pat. No. 4,432,243 to Lowell, for example, a Time Measurement Unit measures the timing of signals transmitted by a transmitter and received by a receiver, and the Time Measurement Unit in turn converts the signals to flow measurements by a processing unit.
It can be appreciated that as the requirements for increased accuracy in these types of timing signals with the Time Measurement Unit is increased, the need for more accurate information relating to the time between waveform signals is required.
A problem arises particularly in the area of digital signal processing in that the bits of words which are used for the readings of the time measurement unit are important. As the need for accuracy increases, the need for each bit of the word, particularly the low order bits, used to obtain the readings is becoming increasingly important.
In digital processing, the magnitude of the signal is usually represented in either a word or double word of computer memory, register bits or the like. These signals may initially be input as an analog signal and subsequently converted to digital signals as they are represented in the word or double word by a series of bits where each bit represents a number. The bit differs from an adjacent bit in the number that the bit represents is different; the bits usually differ by a power of a convenient base, usually two.
In most circumstances, the digital value of the waveform signal has sufficient number of bits so that the desired accuracy in the representation of waveform signal can be achieved. In these cases, the high order bits correspond approximately to the magnitude of the signal waveform while the low order bits provide a relatively smaller contribution to the magnitude of the waveform. Consequently, if the high order bits of the word or double word used for the digital value are stuck and not responding to changing readings for the digital, it is very apparent that these bits are stuck since these bits provide a major contribution to the magnitude of the signal waveform. Consequently, it is relatively easy to determine which of these high order bits are stuck or not responding to the changing digital values of the waveform signal.
In contrast, while the low order bits may provide little relative contribution to the total magnitude of the waveform signal, these low order bits are becoming increasingly important as the accuracy of the magnitude of the digital waveform of the signal waveform is increased. However, it is increasingly difficult to detect a stuck or non-responding bit to changing readings of the low order bits of the signal waveform since these bits provide a relatively small contribution to the magnitude of the waveform. Thus, when these bits are stuck, little change in magnitude is observed.